266 THE OLD MOUTH AND THE NEW. 



functional gill-clefts in front of the hyoid arch metamere or any considerable 

 distance back of the metameres which are now provided with gills in typical fishes. 



VI. THE GUT POUCHES. 



The midgut of the arthropods, in its typical condition, may be regarded as a 

 straight tube with lateral diverticula, or pouches segmentally arranged. In primi- 

 tive Crustacea there may be either a single unbranched diverticulum directed 

 forward and haemally from the anterior end of the gut, cladocera (Fig. 9) ; or one 

 or more pairs of branching lobes, as in phyllopods (Fig. 273), and many cirripeds. 

 (Fig. 275.) 



In the arachnids the gut pouches of the thoracic and abdominal regions 

 become very highly developed, forming one of the most conspicuous features of 

 their internal structure. Their structure and development is clearly seen in 

 Limulus. Here the cephalothoracic yolk mass gradually breaks up into six pairs 

 of lateral lobes. (Figs. 149, 150, lv.l l ~ 6 .) The five anterior pairs form a group 

 by themselves and open into the midgut by a single channel. The sixth pair may 



FIG. 170. Tadpole larva of Zenopus laevis, Daud. After Bles. 6omm. long. 



be distinguished from the others by its larger size, the absence of enclosed cells 

 derived from the haemal blastoderm and by the fact that it opens into the gut by a 

 separate channel. (Fig. 151 Iv.l 6 .) 



In the later stages, a pair of lobes develop from its haemal surface and extend 

 forward and backward, forming the anlage of a special system of haemal gut tubes. 

 (Figs. 151, 154 a. b.) In the adult, the extensive ramifications of the lateral tubes 

 fill the greater part of the cephalothorax, the branches of the posterior haemal 

 tubes, 6, being apparently the only' ones that extend into the abdomen. In 

 the young scorpion, a similar arrangement is seen. (Fig. 179.) The sixth pair, 

 the so-called salivary glands, /././., are large and open, as in Limulus, by separate 

 ducts. The five anterior pairs a.t.L, are reduced to small blind pockets. The 

 haemal lobes h.t.L, are well developed, the large anterior horns extending forward, 

 over the haemal surface of the forebrain. (Fig. 43.) In the branchial region, there 

 are six pairs of pouches, the first corresponding to the comb segment, the next four 

 to the lung books, and the sixth extending backward into the last mesothoracic 

 segment. (Figs. 43, 179.) 



In the pedipalpi, the large haemal lobes of the thorax are united by a transverse 

 anastomosis. In the pycnogonida and in the spiders, the long thoracic diverticulas 

 are unbranched, and may extend a considerable distance into the base of the legs. 

 (Fig. 1 80.) In the spiders, there are also four pairs of abdominal pouches. 



